Fire resistant gypsum composition

ABSTRACT

A composition useful as the core of fire resistant gypsum board and comprising a major amount of set calcined gypsum and minor amounts of whisker fibers, glass fibers and clay.

FIELD OF THE INVENTION

This invention relates to an improved fire resistant composition and,more particularly, to a gypsum composition which is capable of beingused as the core of fire resistant gypsum board.

Fire resistant gypsum board, commonly used as wall and ceilingpartitions, consists of a stiff or rigid core, usually about 1/2 orabout 5/8 inch thick, sandwiched between two paper liners which form theouter surfaces of the board. The composition of the core of gypsum boardis the focus of the present invention.

As its name implies, the main ingredient of the core of gypsum boardderives from gypsum, which is a naturally occurring mineral typicallyfound in old salt-lake beds, volcanic deposits and clay beds in the formof calcium sulphate dihydrate (CaSO₄.2H₂ O). Gypsum is also formed as aby-product of various industrial processes such as, for example, themanufacture of titanium dioxide and the desulfurization of flue gases.Calcining, that is, heating the mineral to about 120° C. drives off 11/2molecules of water, leaving behind calcium sulphate hemihydrate(CaSO₄.1/2H₂ O). Anhydrous calcium sulfate (CaSO₄) can be produced byheating to a temperature of about 163° C. Either of these forms ofgypsum, referred to herein as "calcined gypsum", is capable of reactingwith water to form calcium sulphate dihydrate, which is referred toherein as "set calcined gypsum". Speaking generally, the core of fireresistant gypsum board is made from an aqueous slurry of calcined gypsumwhich has set to form a hard, fairly rigid product.

Fire resistant gypsum board must have certain basic properties in orderto meet accepted standards which dictate whether the board is suitablefor commercial use. Such properties are evaluated in fire resistantassembly tests which have been adaopted by the industry. When exposed tointense heat, such as that generated by fire in a burning building, theboard, which is extensively used for constructing walls and ceilings, isexpected to stay in place for a defined length of time where itfunctions to deter the spread of fire. When subjected to standard fireresistant tests which simulate conditions in a burning building, it isnot unusual for commercially available fire resistant board to remain inplace for one to two hours during which time it is exposed totemperatures as high as 1850° F. In order to perform in this manner, theboard core should resist its tendency to shrink under the influence ofheat because, as it shrinks, it progressively pulls away from thefasteners and supports to which it is fastened and eventually collapses.This allows the fire to spread and attack adjacent parts of thebuilding. In addition, the core should resist cracking as it is heatedand should also exhibit good strength properties at high temperatures.To summarize, fire resistant gypsum board should have good dimensionalstability properties and strength at high temperatures.

This invention relates to the provision of improved fire resistantgypsum board made from a unique core formulation.

REPORTED DEVELOPMENTS

There have been a number of developments which have sought to improvethe fire resistant properties of gypsum board cores. One of thesedevelopments is reported in U.S. Pat. No. 2,526,066 to Croce, whichdiscloses a fire resistant wallboard core comprising set calcinedgypsum, unexpanded vermiculite, glass fibers, and/or asbestos fibers.The fibers serve to prevent spalling and cracking of the core as thevermiculite expands when the core is exposed to fire. The expansion ofthe vermiculite tends to offset the tendency of the gypsum to shrink asthe chemically combined water in the gypsum is driven off by heat.

Another development is reported in U.S. Pat. No. 2,681,863 to Croce andShuttleworth, which discloses gypsum wallboard having enhanced fireresistant properties by virtue of the use of glass fibers which areindividually and uniformly distributed throughout the core thereof. Suchuniform distribution is achieved by initially adding bundles of shortglass fibers which are bonded together by a water soluble or watersoftenable material, to the aqueous gypsum slurry from which the core ismade. As the bonding material dissolves, the glass fibers aredistributed evenly throughout the slurry, eliminating the tendency ofsuch fibers to clump and ball up. Evenly distributed in this way, thefibers better reduce the tendency of the wallboard to crack when it isexposed to the heat of a fire.

U.S. Pat. No. 2,744,022 to Croce and Shuttleworth discloses a gypsumwallboard core which includes glass fibers of the type described in theaforementioned U.S. Pat. No. 2,681,863, and unexpanded vermiculite. Thecombination of unexpanded vermiculite and uniformly distributed glassfibers enhances the resistance of the core to shrinkage and spallingwhen exposed to fire to a greater extent than the aforementioned glassfiber-only composition.

Yet another development is reported in U.S. Pat. No. 3,454,456 toWilley, which discloses fire resistant wallboard having a corecomprising set calcined gypsum and unexpanded vermiculite of acritically defined particle size such that upon exposure to heat thevermiculite expands just enough to compensate for the shrinkage of thegypsum binder. Optional ingredients include mineral fibers, for example,glass and asbestos, and boric acid. Together, the vermiculite andmineral fibers serve to reduce shrinkage and maintain the integrity ofthe core when it is exposed to fire.

Still another development in this field is reported in U.S. Pat. No.3,616,173 to Green and Sundberg, which discloses improved fire resistantwallboard having a core comprising set calcined gypsum, glass fibers,clay, and unexpanded vermiculite. The clay cooperates with otherconstituents of the core to impart thereto improved fire resistantproperties by reducing shrinkage and increasing the temperature at whicha board subjected to heat will fail.

The novel composition of the present invention provides a gypsum boardcore having enhanced resistance to cracking and shrinkage attemperatures above which the otherwise reinforcing glass fiber componentof the core degrades or melts.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided an improved fireresistant gypsum composition and, particularly, a composition which canbe used to excellent advantage as the core of fire resistant gypsumwallboard and which comprises a major amount of set calcined gypsum andminor amounts of glass fibers, whisker fibers and clay. Compositions foruse in the present invention can comprise, for example, about 80 toabout 90 wt.% calcined gypsum; about 0.04 to about 1.0 wt.% glassfibers; about 0.1 to about 2.0 wt.% whisker fibers; and about 0.5 toabout 5.0 wt.% clay.

Whisker fibers contemplated for use in the present invention includeelongate, needle-like crystals of calcium sulfate which aresubstantially insoluble in water or which can be rendered substantiallyinsoluble in water. In the composition of the present invention, suchfibers are dispersed in the set calcined gypsum matrix which comprisesthe bulk of the fire resistant composition. Thus, they retain theirfibrous structure in the end use product.

Important advantages flow from the provision of the present invention.For example, fire resistant wallboard which meets industry-accepted fireresistant tests can be made utilizing whisker fibers, a relativelyinexpensive material. The use of whisker fibers also improves the fireresistant properties of the wallboard in that such fibers continue toeffectively reinforce the core at temperatures as high as 1900° F.,which is about 600° F. higher than the temperature at which glass fibersdegrade. Further savings in material costs can be realized from themanufacture of the wallboard core of the present invention byeliminating the use of vermiculite, a commonly used ingredient inconventional fire resistant gypsum wallboard, without sacrificing thefire resistant properties which are expected of commercial wallboard.And such advantages can be achieved by manufacturing gypsum board of thepresent invention according to conventional techniques utilizingexisting equipment.

DETAILED DESCRIPTION OF THE INVENTION

The essential ingredients of the fire resistant gypsum board core of thepresent invention are set calcined gypsum, glass fibers, clay andwhisker fibers.

The ingredient present in the core of the invention in the greatestamount is set calcined gypsum, that is, calcium sulfate dihydrate formedby the hydration of anhydrous calcium sulfate or of calcium sulfatehemihydrate, either of which can be formed by calcining gypsum.Hydration is typically effected from a water slurry containing thecalcined gypsum and other ingredients comprising the core in accordancewith usual gypsum board manufacturing techniques. The set calcinedgypsum functions to bind together the core ingredients in the form of asolid continuous mass.

The calcined gypsum component should not be so alkaline as to have adegrading effect upon the glass fibers of the core composition. Inpreferred form, a 100 g sample of the calcined gypsum mixed with anequal amount of water forms a mixture which has a pH of about 7 to about9, most preferably in excess of 7 and less than 9.

The glass fiber component of the gypsum board core functions to reducecracking and spalling of the core when it is exposed to heat. It ispreferred to use drawn textile glass fibers of the type described inaforementioned U.S. Pat. Nos. 2,681,863 and 2,744,022 to Croce andShuttleworth. The manufacturing and functional improvements andadvantages obtained by utilizing such glass fibers relative to blownglass fibers or other natural synthetic fibers are described in theaforesaid Croce/Shuttleworth patents, the disclosures of which, as theyrelate to the description of the drawn textile glass fibers, areincorporated herein by reference.

Briefly described, the drawn textile glass fibers are produced ascontinuous individual filaments having a diameter of, for example, about0.0002 to 0.001 inch. Individual fine filaments are grouped into strandshaving from 100 to several hundred individual filaments which areprotectively coated to prevent abrading between grouped filaments.Preferred glass fiber strands for use in this invention comprise thosewhich have been coated with a relatively weak bonding type material,such as, for example, starch or other water softenable or solublecoating material.

Prior to combining the loosely bonded textile glass fibers with theother components which comprise the core composition, they arepreferably cut into short lengths, such as, for example, about 1/8 toabout 1 inch, providing short sections or bundles of fibers. Upon addingthe short sections or bundles of the fibers to the aqueous slurrycomposition from which the core of the gypsum board is formed, thebonding or coating material softens or dissolves. As the slurry ismixed, the glass fibers which comprise the bundle are separated intoindividual filaments which are completely dispersed in the slurry. Afterthe slurry has been allowed to set, there is obtained a core havingshort, substantially straight, resilient, flexible, individual textileglass filaments distributed randomly therethrough.

The clay component of the gypsum board core improves the core's abilityto resist shrinking upon exposure to heat. Examples of clays which canbe used in the practice of the present invention are described inaforementioned U.S. Pat. No. 3,616,173 to Green and Sundberg, thedisclosure of which, as it relates to clays, is incorporated herein byreference.

Examples of types of clays that can be used are bentonite--comprisedmainly of the clay mineral montmorillonite; attapulgite--clays whichcontain magnesium aluminum silicates; and kaolinitic clays--includingfor example, kaolin (also referred to as china or paper clays), ballclay, fireclay, and flint clay--which clays are comprised predominatelyof the clay mineral kaolinite. Other of the various types of clays whichcontain mixtures of various proportions of clay minerals, such as forexample illite, chlorite, kaolinite and montmorillonite, as well asnonclay materials, may also be used. It is preferred to use kaoliniticclays.

The whisker fiber component of the gypsum board core of the presentinvention comprises needle-like crystals of calcium sulfate which aresubstantially insoluble in water or which are capable of being renderedsubstantially insoluble in water. There are two basic differencesbetween the whisker fiber component and the calcined gypsum componentfrom which the core is made. Whisker fibers are generally long and thin,whereas the calcined gypsum component primarily comprises irregularlyshaped block-like crystals. Whisker fibers for use in the presentinvention are substantially insoluble in water or may be rendered sowhereas the calcined gypsum component readily rehydrates to form calciumsulfate dihydrate.

The whisker fiber component of the core functions to reduce cracking andspalling of the core when it is exposed to elevated temperatures,including temperatures at which the glass fiber component loses itsfunctional properties as it melts away. In this connection, the whiskerfibers can continue to function at temperatures as high as 1900° F. ormore which is about 600° F. higher than that at which glass fibers losetheir effectiveness.

Whisker fibers are well known in the art. See, for example, thefollowing U.S. Pat. No. 4,152,408 (whisker fibers as reinforcement fororganic polymeric resins, hydraulic cements, wood pulp and paperproducts); No. 4,029,512 (fibrous insoluble calcium sulfate anhydrite asreinforcement for hydraulic cement matrices such as Portland cement andorganic polymeric matrices such as polyester and polypropylene); No.3,961,105 (waterproofed whisker fibers, no disclosed use); and No.3,822,340 (whisker fibers as replacement for asbestos for insulating andfireproofing of buildings; use in structural units such as beams,sheathing, flooring and the like; as reinforcement for glass fibers; usein a fire resistant 100% whisker fiber molded board product).Additionally, the following publications disclose various types ofwhisker fibers and applications thereof; U.K. Patent Application No.2,053,874A (fibrous calcium sulfate useful in construction materialssuch as panels, heat insulating material, and core material); JapanesePatent Application No. 77-63755 (incombustible gypsum fiberboardcomprising a major amount of bulky needle crystals of calcium sulfatehemihydrate, and minor amounts of glass fibers, paper pulp,polyacrylamide and silicon); Japanese Patent Application No. 76-80937(calcium sulfate hemihydrate needle crystals, no use disclosed);Japanese Patent Application No. 78-61474 (acicular gypsum crystals asreinforcement for thermoplastic resins such as polypropylene); JapanesePatent Application No. 77-48667 (bulky acicular calcium sulfateanhydrite crystals, that is, gypsum whiskers as material fornon-flammable building material such as sheeting and boards and as afiller for plastics); and Japanese Patent Application No. 76-137037(needle-like crystal fibers of calcium sulfate hemihydrate as fillermaterial for synthetic resins, paint lacquer and the like). It isevident from the foregoing that whisker fibers have been used in a widevariety of applications but their use in the core of fire resistantgypsum board, as described herein, has not heretofore been known.

The manufacture of whisker fibers is well known in the art. See, forexample, the following U.S. Pat. Nos.: 3,822,340; 3,961,105; 4,029,512;and 4,152,408. Additionally, the following publications disclose themanufacture of "needle crystals" and fibers of calcium sulfate which mayalso be used in the practice of the present invention: U.K. PatentApplication No. 2,053,874 A; and Japanese Patent Application Nos.:77-63755; 76-80937; 78-61474; 77-48667; and 76-137037.

Generally speaking, whisker fiber manufacture begins by heating anaqueous slurry of calcium sulfate dihydrate (gypsum) under pressure.Whisker fibers usually form from the slurry as elongated thin crystalsof calcium sulfate hemihydrate (CaSO₄.1/2H₂ O), although recovery ofwhisker fibers in the dihydrate form is reported in aforementionedJapanese Patent Application No. 77-63755. Once recovered, the whiskerfibers may be treated further by heating to drive off the remainingchemically bound water. This leads to the formation of soluble anhydrouscalcium sulfate whisker fibers. Heating still further renders the fibersinsoluble in water. Thus, whisker fibers may be recovered in a dihydrateform (CaSO₄.2H₂ O), a hemihydrate form (CaSO₄.1/2H₂ O), a solubleanhydrite form (CaSO₄), and an insoluble anhydrite form (CaSO₄).Preferred whisker fibers for use in the practice of the presentinvention are insoluble calcium sulfate anhydrite whisker fibers.

Whisker fibers which are otherwise soluble in water may be coated withvarious materials which waterproof or otherwise render the coated fibersinsoluble in water. Examples of such materials include waxes, hydrolizedproteins, and polycarboxylic acids.

Preferred whisker fibers for use in the practice of the presentinvention are calcium sulfate whisker fibers having: a length todiameter ratio of about 400:1 to about 5:1, preferably about 200:1 toabout 10:1, the length to diameter ratio most preferably averaging fromabout 20:1 to about 30:1; a length of about 10 to about 400 micrometers,preferably about 20 to about 200 micrometers, the length most preferablyaveraging from about 35 to about 45 micrometers; and a testingconsistency of about 100 to about 800 grams of water to about 100 gramswhisker fibers, preferably about 200 to about 350 grams of water toabout 100 grams whisker fibers.

With respect to optional ingredients of the core of the presentinvention, vermiculite, which is used in fire resistant wallboard thatis widely sold, is such a material. Vermiculite comprises aheat-expandable mineral of volcanic origin which, upon heating, expandsin a manner such that it tends to compensate for or offset the inherentshrinkage of the set calcined gypsum component of the core as thechemically combined water of this component is driven off by the heat ofa fire. Accordingly, it functions to deter shrinkage of the core andimpart dimensional stability to the core as the wallboard is subjectedto heat. Ideally, the quantity of vermiculite present in the compositionshould be such that when it expands it will substantially equal theshrinkage of the set calcined gypsum component as it is heated and asits combined water is driven off.

Although vermiculite can be used in the core of the present invention,it should be considered as an optional ingredient. Evaluations haveshown there can be formulated vermiculite-free cores which haveexcellent fire resistant properties that are at least as good as coresformulated with vermiculite.

Other materials can be included optionally in the core and theformulations from which the core is made to impart desired propertiesthereto and/or facilitate manufacturing. Some examples of such materialsinclude foaming agents, accelerating agents, dispersing agents, and coreadhesives.

The components comprising the core can be mixed according to knownmethods and gypsum board and the core thereof formed according to knowntecchniques. Speaking generally, this initially involves metering thedry ingredients and water into a mixer and forming an aqueous mixturetherefrom. The density of the core can be controlled by adding to theaqueous mixture foam formed from a dilute aqueous solution of a surfaceactive material, for example, ammonium or sodium lauryl sulfonate. Thedilute aqueous solution can be foamed utilizing air pressure. Thequantity of foam added to the aqueous mixture determines the amount ofvoids or cells present in the resulting core inasmuch as the foamdisplaces with air other of the core ingredients to form cells or voidsin the core. The foam, water and other ingredients are mixed thoroughlyto form a pourable aqueous slurry which is dispensed through one or moreoutlets at the bottom of the mixer onto a moving facing sheet or coversheet. Another facing sheet is placed on top of the slurry to sandwichit between two moving facing sheets which are the paper facings of theresultant gypsum board. Facing sheets are usually paper but may also beof non-woven or woven fiberglass mats, plastic scrims and the like.

The thickness of the resultant board is controlled by a forming roll andthe edges of the board are formed by appropriate mechanical deviceswhich continuously score, fold and glue the overlapping edges of thefacing sheets. Additional guides are used to maintain thickness andwidth as the "setting" slurry travels on a moving belt. Desired lengthsof board are cut in a continuous operation. Evaporation from the core ofexcess water which is not involved in the hydration of the calcinedgypsum is generally accelerated by heating the board.

With respect to the amounts and proportions of ingredients comprisingthe set composition of the present invention, as mentioned above,calcined gypsum comprises at least 50 wt. % of the set composition, withthe other ingredients being present in minor, but functional amounts. Infabricating cores of fire resistant gypsum board in accordance with thepresent invention, it is recommended that the core comprise the setproduct of at least about 80 wt. % of calcined gypsum, at least about0.04 wt. % glass fibers, at least about 0.1 wt. % whisker fibers, and atleast about 0.5 wt. % clay. (The term "wt. %" means weight percent basedon the total weight of the dry ingredients prior to the addition ofwater to form a settable slurry.) Recommended ranges of amounts are setforth below.

    ______________________________________                                                                  Preferred Range,                                    Ingredients                                                                              Broad Range, wt. %                                                                           wt. %                                               ______________________________________                                        calcined gypsum                                                                          about 80 to about 90                                                                         about 83 to about 88                                glass fibers                                                                             about 0.04 to about 1.0                                                                      about 0.1 to about 0.40                             whisker fibers                                                                           about 0.1 to about 2.0                                                                       about 0.4 to about 1.0                              clay       about 0.5 to about 5.0                                                                       about 1.5 to about 3.5                              ______________________________________                                    

If vermiculite is included in the core, it will generally beadvantageous to include at least about 0.5 wt. %, for example, about 0.5to about 1.5 wt. %, and preferably about 0.8 to about 1.2 wt. %. Otheradditives such as accelerating agents, dispersing agents (for example,sodium lignosulfonate) and core adhesives (for example, corn starch) aregenerally added in addition to the above ingredients in amountswell-known in the art in order to impart desirable properties to thegypsum board and/or to facilitate the manufacturing thereof. In general,the amounts of such additives generally comprise collectively about 0.5to about 2.0 wt. % of the core.

Operating within the above parameters, it is possible to form animproved fire resistant gypsum board core having a density of about 44to about 50 lbs./cu. ft. and the amount and proportion of ingredientscomprising the core being such that a core having a thickness of about0.5 inch has a shrink resistance of no less than about 75% and cracks nomore than about 6 inches/24 square inches after heating to about 1800°F. for about 2 hours.

EXAMPLES

Examples which follow are illustrative of the fire resistance propertiesof various gypsum board cores within the scope of the present invention.Comparative examples are included also. Two fire resistance propertieswere measured for all core samples tested: shrink resistance andcracking.

Shrink resistance, as used herein, of a gypsum board core is a measureof the percentage of the core's width that remains after a core samplehas been heated to a defined temperature over a defined period of time.The extent to which gypsum board resists shrinkage when it is subjectedto heat is related to the dimensional stability of its core and is anindication of how long the gypsum board will remain in place before itpulls away from its fasteners and joist supports because of shrinking.

Cracking, as used herein, of a gypsum board core is a measure of thetotal length of cracks which develop in a core sample heated to adefined temperature over a defined period of time. The extent to which acore cracks when it is subjected to heat is an indication of how wellthe gypsum board will serve as a barrier to the spread of fire.

In all of the examples which follow, the gypsum, glass fiber, whiskerfiber and clay components that were used are as follows:

    ______________________________________                                        COMPONENTS  DESCRIPTION                                                       ______________________________________                                        calcined gypsum                                                                           commercially available calcined gypsum,                                       generally comprising about 90% calcium                                        sulfate hemihydrate and the remaining                                         10% comprising various impurities such                                        as, for example, limestone, shale, sand                                       and clay                                                          glass fibers                                                                              strands of drawn textile glass fibers,                                        the fibers having a diameter of about                                         0.00065 to about 0.0007 inch, the strands                                     having a length of about 0.5 inch and                                         loosely bonded together by starch and                                         sold by Owens Corning Fiberglass                                  whisker fibers                                                                            insoluble calcium sulfate anhydrite                                           whisker fibers having a length to                                             diameter ratio within the range of                                            about 200:1 to about 10:1, a length                                           within the range of about 20 to about                                         200 micrometers and a test consistency                                        of about 200 to about 350 ml of water                                         per 100 grams of whisker fibers                                   clay        ASP 602, a water washed kaolin clay                                           having an average particle size of 0.8                                        micron, a specific gravity of 2.58, and                                       sold by Minerals and Chemical Phillip                                         Corporation of America                                            ______________________________________                                    

In particular, tests to determine shrink resistance and cracking wereconducted by preparing core samples having a length of 8 inches, a widthof 3 inches and a thickness of 1/2 inch. Samples were prepared by mixingthe dry components comprising the core, adding the premixed componentsto a quantity of water sufficient to achieve a core density of about 46lbs. per cubic foot, and then mixing for one minute with mechanicalagitation. The resulting mixture was poured into a mold and whensufficiently hardened, the cast was removed from the mold and cut intofour samples, each of which was 3 inches wide, 8 inches long and 1/2inch thick. After being thoroughly dried at 110° F., the four samplescut from the cast were positioned within a muffle furnace, each samplebeing placed upright therein on its 8-inch long, 1/2-inch thickness edgein such a way that the samples did not touch each other and roughlydefined the sides of an 8-inch by 8-inch square.

The oven and samples were at room temperature when the samples wereplaced therein. The furnace was then turned on and set to 1800° F. Aftertwo hours, the furnace was turned off and allowed to cool undisturbeduntil the next day. The samples were then removed and evaluated forshrinkage and cracking.

Shrink resistance was determined by measuring the width of each sampleafter heating as described above, and calculating the percent widthremaining. Length was not used to calculate percent reduction becausecracks which developed in the samples in response to heating wereobserved to run across the width of a sample. This causes an apparentreduction in shrinkage by increasing sample length.

Cracking was determined by measuring the length of each crack in a givensample and calculating the total length of the cracks by summing thelengths of each of the cracks. Cracks of 1/4 inch or less were notmeasured.

Mean shrink resistance and cracking values are set forth in the tablebelow for the core compositions of Example Nos. 1 to 8 which areillustrative of the invention. The same type values are set forth in thetable for the comparative core compositions of Example Nos. 1-C to 4-C.All values expressed represent the average of values from 4 samples. Thedensity of each of the cores of the examples was approximately 46 lb/cu.ft.

    ______________________________________                                        CORE INGREDIENTS, wt. %                                                       Ex-  Cal-                    TEST VALUES                                      am-  cined                       Shrink                                       ple  Gyp-    Glass   Whisker     Resis-  Cracking                             No.  sum     Fibers  Fibers Clay tance (%)                                                                             (inches)                             ______________________________________                                        1-C  100     0       0      0    73.5 ± 1.0                                                                         10.1 ± 1.8                        2-C  99.78   0.22    0      0    71.5 ± 1.0                                                                         6.9 ± 1.8                         3-C  99.12   0       0.88   0    72.5 ± 1.0                                                                         6.7 ± 1.8                         4-C  99.5    0.1     0.4    0      72 ± 1.0                                                                         6.0 ± 1.8                         1    99.27   0.05    0.14   0.54   73 ± 1.7                                                                         7.6 ± 2.7                         2    99.09   0.22    0.14   0.55   73 ± 1.7                                                                         4.0 ± 2.7                         3    97.61   0.05    0.14   2.2    83 ± 1.7                                                                         7.7 ± 2.7                         4    97.44   0.22    0.14   2.2    84 ± 1.7                                                                         1.4 ± 2.7                         5    98.85   0.05    0.55   0.55   74 ± 1.7                                                                         3.9 ± 2.7                         6    98.68   0.22    0.55   0.55   71 ± 1.7                                                                         2.7 ± 2.7                         7    97.21   0.05    0.54   2.2    82 ± 1.7                                                                         2.0 ± 2.7                         8    97.03   0.22    0.55   2.2    80 ± 1.7                                                                         1.3 ± 2.7                         ______________________________________                                    

As the foregoing examples illustrate, the best shrink resistance andleast cracking was attained in those core samples tested which containedcalcined gypsum, glass fibers, whisker fibers and clay.

The example which follows is illustrative of a preferred fire resistantgypsum board core formulation.

EXAMPLE 9

A core of fire resistant gypsum board illustrative of a preferredformulation of the invention and having a density of about 46 lbs./cu.ft. and a board weight of about 1850 lbs./sq. ft. may be prepared fromthe set product of the following components (of the same description asset forth for the preceding examples) in the amounts indicated.

    ______________________________________                                                            Wt. %                                                     ______________________________________                                        Calcined gypsum       about 94.9;                                             glass fibers          about 0.2;                                              whisker fibers        about 0.5;                                              clay                  about 3.3;                                              dispersing agent      about 0.1;                                              (sodium lignosulfonate)                                                       foaming agent         about 0.5; and                                          (sodium lauryl sulfonate)                                                     core adhesive         about 0.5.                                              (corn starch)                                                                 ______________________________________                                    

To summarize, the incorporation of relatively inexpensive calciumsulfate whisker fibers in gypsum compositions provides a gypsum boardcore having enhanced resistance to shrinkage and cracking attemperatures above which boards reinforced with glass fibers only fail.Material cost savings are further realized by eliminating vermiculite, acommonly used ingredient in gypsum boards used as conventional fireresistant wallboard. These advantages are achieved in the manufacture offire resistant gypsum board of the instant invention utilizing existingequipment according to conventional techniques.

We claim:
 1. A core of fire-resistant gypsum board comprising the setproduct of at least about 80 wt.% of calcined gypsum, at least about0.04 wt.% glass fibers, at least about 0.1 wt.% whisker fibers and atleast about 1.5 wt.% clay.
 2. A core according to claim 1 comprising theset product of:

    ______________________________________                                                       Wt. %                                                          ______________________________________                                        calcined gypsum  about 80 to about 90;                                        glass fibers     about 0.04 to about 1.0;                                     whisker fibers   about 0.1 to about 2.0; and                                  clay             about 1.5 to about 5.0.                                      ______________________________________                                    


3. A core according to claim 1 comprising the set product of:

    ______________________________________                                                       Wt. %                                                          ______________________________________                                        calcined gypsum  about 83 to about 88;                                        glass fibers     about 0.1 to about 0.4;                                      whisker fibers   about 0.4 to about 1.0; and                                  clay             about 1.5 to about 3.5.                                      ______________________________________                                    


4. A core according to claim 1 and having a density of about 44 to about50 lbs./cu. ft., the amounts and proportions of ingredients comprisingthe core being such that a core having a thickness of about 0.5 inch hasa shrink resistance of no less than 75% and cracks no more than 6inches/24 sq. in. after heating to about 1800° F. for about 2 hours. 5.A core according to any of claims 1, 2, 3 or 4, wherein said whiskerfibers are water insoluble calcium sulfate anhydrite.
 6. A coreaccording to claim 1 wherein said whisker fibers have a testingconsistency of about 100 to about 800 grams of water per 100 grams ofwhisker fibers and a length to diameter ratio of about 400:1 to about5:1.
 7. A core according to claim 6 wherein said whisker fibers have anaverage length to diameter ratio of about 20:1 to about 30:1.
 8. A coreaccording to claim 1 wherein said whisker fibers have a length of about10 to about 400 micrometers.
 9. A core according to claim 8 wherein saidwhisker fibers have an average length of about 35 to about 45micrometers.
 10. A core according to claim 1 wherein said whisker fibershave a testing consistency of about 200 to about 350 grams of water to100 grams of fibers and a length to diameter ratio of about 200:1 toabout 10:1.
 11. A core according to claim 10 wherein said whisker fibershave a length of about 20 to 200 micrometers.
 12. A core according toclaim 1 which is substantially free from heat expandable minerals.
 13. Acore according to claim 1 including at least about 0.5 wt. %vermiculite.
 14. A core according to claim 2 including about 0.5 toabout 1.5 wt. % vermiculite.
 15. A core according to claim 3 includingabout 0.8 to about 1.2 wt. % vermiculite.
 16. A core according to claim1 wherein the clay is kaolin clay.
 17. A process for making a core ofgypsum board comprising preparing a mixture of water, a major amount ofcalcined gypsum and minor amounts of glass fibers, whisker fibers andclay, and forming and setting said mixture into said core, wherein on adry basis, said mixture comprises at least about 80 wt.% of saidcalcined gypsum, at least about 0.04 wt.% of said glass fibers, at leastabout 0.1 wt.% of said whisker fibers, and at least about 1.5 wt.% ofsaid clay.
 18. The process of claim 17, wherein the whisker fibers arewater insoluble calcium sulfate anhydrite having a length to diameterratio of about 400:1 to about 5:1, a testing consistency of about 100 toabout 800 grams of water to about 100 grams of whisker fibers, and alength of about 10 to about 400 micrometers.
 19. The process of claim17, wherein the whisker fibers have a length to diameter ratio of about200:1 to about 10:1, a testing consistency of about 200 to about 350grams of water to about 100 grams of whisker fibers and a length ofabout 20 to 200 micrometers.
 20. The process of claim 19, wherein thewhisker fibers have an average length to diameter ratio of about 20:1 toabout 30:1 and an average length of about 35 to about 45 micrometers.21. A core according to claim 2 having a density of about 44 to about 50lbs./cu. ft., the amounts and proportions of ingredients comprising thecore being such that a core having a thickness of about 0.5 inch has ashrink resistance of no less than 75% and cracks no more than 6inches/24 sq. in. after heating to about 1800° F. for about 2 hours. 22.A core according to claim 21 comprising the set product of:

    ______________________________________                                                       Wt. %                                                          ______________________________________                                        calcined gypsum  about 83 to about 88;                                        glass fibers     about 0.1 to about 0.4;                                      whisker fibers   about 0.4 to about 1.0; and                                  clay             about 1.5 to about 3.5.                                      ______________________________________                                    


23. A core according to claim 21 or 22 wherein said whisker fibers arewater insoluble calcium sulfate anhydrite having a testing consistencyof about 100 to about 800 grams of water per 100 grams of whisker fibersand a length to diameter ratio of about 400:1 to about 5:1.